Electric machines may be operated as an electric motor or an electric generator. In either conversion: electricity to mechanical energy or mechanical energy to electricity, there are losses which lead to heating of the electric machine. Often, a housing in which the electric machine is disposed serves as a heat sink, in which case, the interface between the electric machine and the housing is designed to promote heat transfer. Sometimes, the housing has fins on its outer surface to increase heat transfer to the environment. In some applications, the electric machine is in the vicinity of elements which are at a higher temperature than the housing such that energy is transferred into the electric machine further exacerbating overheating of the machine and putting an additional strain on cooling the machine. In such applications, decreasing heat transfer between the electric machine and the housing is desirable. One such configuration in which a hot element is proximate the housing of the electric machine is an electronically controlled turbocharger (ECT) in which the turbine housing may attain temperatures exceeding 1000° F. As the electric machine housing is cooler than that, the turbine housing radiates to the electric machine housing.
To overcome at least one problem in the prior art, an electric machine is provided that has a housing having an inner surface with at least part of the inner surface forming a substantially cylindrical cavity, a first end cavity, and a second end cavity; first and second bearings mounted in the housing; a shaft mounted within the first and second bearings; a rotor affixed to the shaft and located between the first and second bearings; and a stator disposed in the housing. The stator has a substantially cylindrical outer surface. There is a gap between the substantially cylindrical cavity of the housing and the substantially cylindrical outer surface of the stator. The gap exists over a majority of the substantially cylinder outer surface. The first end cavity is in fluidic communication with the gap. The second end cavity is in fluidic communication with the gap.
In some embodiments, the assembly may further include a sleeve inserted in the gap wherein the sleeve is comprised of a low thermal conductivity material.
In some embodiments, the inner surface of the housing has first and second circumferential grooves with a first split ring disposed in the first circumferential groove and a second split ring disposed in the second circumferential groove wherein the substantially cylindrical outer surface of the stator contacts inner surfaces of the first and second split rings. In another embodiment, the inner surface of the housing has first, second, and third axial grooves, with a first rod disposed in the first groove; a second rod disposed in the second groove; and a third rod disposed in the third groove wherein the substantially cylindrical outer surface of the stator contacts the inner surfaces of the first, second, and third rods. The rods may have a polygonal cross-sectional shape.
In some embodiments, multiple housing pieces couple together to form the housing.
In some alternatives, at least a portion of the inner surface of the housing and at least a portion of the substantially cylindrical outer surface of the stator have an anti-wetting coating applied. In other alternatives, a reflective coating is provided on at least: a portion of the inner surface of the housing or a portion of the substantially cylindrical outer surface of the stator.
The disclosure may be advantageously used in an electronically-controlled turbocharger (ECT) which includes: a shaft onto which a turbine wheel, a compressor wheel, and a rotor of an electric machine is affixed; a housing having a substantially cylindrical inner surface, a first end cavity, and a second end cavity; first and second bearings mounted onto the shaft with the housing mounted on the first and second bearings; and a stator disposed in the housing. The stator has a substantially cylindrical outer surface. A gap exists between the substantially cylindrical inner surface of the housing and the substantially cylindrical outer surface of the stator over a majority of the substantially cylinder outer surface of the stator. The gap is in fluid communication with the first end cavity. The gap is in fluid communication with the second end cavity.
In some embodiments, the ECT may have a low thermal-conductivity sleeve inserted in the gap.
In some embodiments, the cylindrical inner surface of the housing has first and second circumferential grooves with a first split ring disposed in the first circumferential groove and a second split ring disposed in the second circumferential groove. The outer surface of the stator contacts inner surfaces of the first and second split rings. In an alternative embodiment, the cylindrical inner surface of the housing has first, second, and third axial grooves with a first rod disposed in the first groove, a second rod disposed in the second groove, and a third rod disposed in the third groove.
In some alternatives, at least three standoffs extend inwardly from the inner surface of the housing and the outer surface of the stator is supported in the housing by the standoffs.
Also disclosed is an electric machine having a housing; first and second bearings mounted in the housing; a shaft mounted within the first and second bearings; a rotor affixed to the shaft and located between the first and second bearings; a stator disposed within the housing; and material disposed between the stator and the housing. At least a portion of an inside surface of the housing is substantially cylindrical; at least a portion of an outer surface of the stator is substantially cylindrical; and, in some embodiments, the material is a sleeve having low thermal-conductivity. In one non-limiting example, the material is a ceramic. In some embodiments the material is sprayed onto at least a section of the substantially cylindrical inside surface of the housing, such as with thermal-plasma spraying. The material has low thermal conductivity and/or high reflectivity.
According to embodiments disclosed herein, energy transferred to the electric motor via radiation and conduction from an external, high-temperature element, such as the turbine housing of a turbocharger, can largely be mitigated by insulating the stator or by dripping coolant between the stator and housing to extract energy.